A chemical reaction is in which the bonds are broken within reactant molecules, and new bonds are formed within product molecules in order to form a new substance.
The basis for different types of reactions is the product formed, the changes that occur, the reactants involved and so on. Different types of reactions are
The reactions in which two or more substances combine to form a single new substance are called combination or synthesis reactions.
Combination Reaction
Example:
Burning of coal: C(s) + O2(g) → CO2(g)
The reaction in which a single compound breaks up into two or simpler substances is known as the Decomposition reaction.
Decomposition Reaction
Decomposition reactions can be classified into three types:
Silver bromide also decomposes in the same way
The decomposition of a compound with light is called Photolysis. The above reactions are used in black and white photography.
Note:
1. All the decomposition reaction requires energy i.e. these reactions are Endothermic reactions. These reactions are used in Extraction of metals.
2. A decomposition reaction is called the opposite of a combination reaction. This can be supported by the following reactions:
Combination reaction:Decomposition reaction:
The chemical reactions in which one element takes the place of another element in a compound are called Displacement Reactions.
Displacement ReactionExample: Reaction of iron nails with copper sulphate solution.
In this reaction, iron has displaced copper from copper sulphate solution.
Question 1: In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.
The reactions in which two compounds react to form two different compounds by mutual exchange of ions are called double displacement reactions.
Example of Double Displacement ReactionExample: On mixing a solution of barium chloride with sodium sulphate, a white precipitate of barium sulphate is immediately formed. These reactions are ionic in nature. The reactants change into ions when dissolved in water and there is an exchange of ions in solution. This results in the formation of product molecules.
1. Precipitation Reaction
A chemical reaction that involves the formation of an insoluble product (precipitate; solid) is called a Precipitation reaction.
A precipitation reaction occurs when a solution, originally containing dissolved species, produces a solid, which generally is denser and falls to the bottom of the reaction vessel. The most common precipitation reactions occurring in aqueous solution involve the formation of an insoluble ionic compound when two solutions containing soluble compounds are mixed.
Example: Consider what happens when an aqueous solution of NaCl is added to an aqueous solution of AgNO3. The first solution contains hydrated Na+ and Cl− ions and the second solution, Ag+, and NO3− ions.
NaCl(s) → Na+(aq) + Cl−(aq)
AgNO3(s) → Ag+(aq) + NO3−(aq)
When mixed, a double displacement reaction takes place, forming the soluble compound NaNO3 and the insoluble compound AgCl. In the reaction vessel the Ag+ and Cl− ions combine, and a white solid precipitated from the solution. As the solid precipitates, the Na+ and NO3− ions remain in the solution.
The overall double displacement reaction is represented by the following balanced equation:
NaCl(aq) + AgNO3(aq) → AgCl(s) + NaNO3(aq)
2. Neutralisation Reaction
Example:
Oxidation means gaining oxygen or removal of Hydrogen in a chemical reaction
Reduction means addition of Hydrogen or loss of oxygen in a chemical recations
What is an Oxidising agent?
The substance which gives oxygen or removes hydrogen for oxidation is called oxidising agent and the substance which gains oxygen during the reaction is said to be oxidised.
What is a Reducing agent?
The substance which gives hydrogen or removes oxygen for reduction is called the reducing agent. The substance which gains hydrogen during the reaction is said to be reduced.
Electron movement in Oxidising and Reducing Agent
Those reactions in which oxidation and reduction (both) occur simultaneously are called Redox reactions.
Do you know? 🤔
In the name Redox, the term 'red' stands for reduction and 'ox' stands for oxidation.
3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g)
(i) Iron metal is getting oxidised.
(ii) Water is getting reduced.
(iii) Water is acting as reducing agent.
(iv) Water is acting as oxidising agent.
Question 2: A shiny brown coloured element 'X' on heating in the air becomes black in colour. Name the element 'X' and the black coloured compound formed.
An element of heating in air changes in its oxide. The brownish element which forms black oxide is copper.
The name of the element is Copper (Cu)
Name of black compound: Copper(II) oxide, (CuO)
Reaction:
Oxidation has a damaging effect on metals as well as on food. The damaging effect of oxidation on metals is studied as corrosion and that on food is studied as rancidity.
There are two common effects of oxidation reactions areas:
Corrosion is the process of deterioration of metals as a result of its reaction with air, moisture, and acids (Present in the environment) surrounding it.
Different conditions of an iron nail rusting
The corrosion causes damage to buildings, bridges, ships and many other articles specially made of iron.
Iron corrodes readily when exposed to moisture and gets covered with a brown flaky substance called rust.
Rusted Chains
This is called rusting of iron, Rust is a hydrated iron (III) oxide Fe2O3· 2H2O.
Rusting of iron takes place under the following conditions:
It has been observed that:
Methods used to prevent Rusting of Iron are as follows:
Alloyed Bar
Galvanizing
Coating and Painting
Copper corrosion
Black Silver Sulfide Deposition
Fresh foods containing fats and oils smell and taste pleasant but when they remain exposed to the air for a long time, smell and taste change to unpleasant. It is said that food has become rancid.
OR
It is due to the oxidation of fats and oils, butter, ghee, boiled rice, etc, after prolonged exposure to air i.e. The condition produced by the aerial oxidation of fats and oils in foods marked by unpleasant smell and taste is called rancidity.
Rancidity in food products
The two common antioxidants are:
BHA (Butylated Hydroxy Anisole)
BHT (Butylated Hydroxy Toluene)
Difference Between Endothermic and Exothermic Reactions
Q.1: What is the method of balancing chemical equation?
Ans: Hit and trial method is used for balancing simple chemical equations. In this method, coefficients before the symbols/formulae of the reactants and products are adjusted in such a way that the total number of atoms of each element on both the sides becomes equal.
Q.2: In the equations given below, state giving reasons, whether substances have been oxidised or reduced.
(i) PbO + CO –> Pb + CO2
(ii) H2S + Cl2–>2HCl + S.
Ans:
(i) Carbon monoxide is oxidised as it gains oxygen.
(ii) Chlorine is reduced as it gains hydrogen.
Q.3: What are the different ways can make more informative about the chemical equation?
Ans:
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1. What is a chemical reaction? | ![]() |
2. What are the different types of chemical reactions? | ![]() |
3. What is corrosion and how does it relate to chemical reactions? | ![]() |
4. What is rancidity and how does it relate to chemical reactions? | ![]() |
5. What is the difference between endothermic and exothermic reactions? | ![]() |
78 videos|508 docs|153 tests
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